1. Field of the Invention
The present invention relates to a method of manufacturing a crystalline film and a crystalline-film-layered substrate, and to a method of manufacturing a thermoelectric conversion element using the foregoing method. The present invention also relates to a thermoelectric conversion element including a crystalline film obtained by epitaxial growth as a thermoelectric conversion layer.
2. Description of the Related Art
Thermoelectric power generation is a technology for directly converting thermal energy into electric energy with the use of the Seebeck effect, a phenomenon in which a temperature difference given to opposing ends of a substance causes a thermal electromotive force in proportion to the temperature difference. The thermal electromotive force can be taken out as electric power by connecting a load thereto and forming a closed circuit. This technology has been in practical use as power supplies for remote areas, power supplies for aerospace use, power supplies for military use, and so forth.
Thermoelectric cooling is a technology for causing heat absorption at a junction of two substances with the use of the Peltier effect, a phenomenon in which, when a current is passed through two substances having carriers with different signs, for example, a p-type semiconductor and an n-type semiconductor, that are connected thermally in parallel and electrically in series, the difference in the signs of the carriers reflects the difference in the directions of the heat flow. This technology has been in practical use as local cooling devices such as for cooling electronic devices in a space station, wine coolers, and the like.
Characteristics of a thermoelectric conversion material are evaluated by a figure of merit Z, or a figure of merit ZT that is made dimensionless by multiplying the figure Z by an absolute temperature. The figure of merit ZT is an index represented as ZT=S2/ρκ by S, ρ, and κ of a substance, where S is Seebeck coefficient, ρ is electric resistivity, and κ is thermal conductivity. Using this index as a criterion, a material that has good thermoelectric conversion performance is currently pursued.
New applications of thermoelectric conversion elements, for example, local cooling for electronic devices such as mobile telephones and personal computers, power generating apparatuses for wearable electronic devices, or the like, will indispensably require a thermoelectric conversion element in which a thermoelectric conversion material is formed on a substrate in the form of a thin film. In the thermoelectric conversion element including a substrate and a thermoelectric conversion layer formed thereon, the heat conduction by the substrate becomes dominant over the heat conduction by the thermoelectric conversion layer. Therefore, even if a great temperature difference is produced at both ends of the thermoelectric conversion layer made of a thermoelectric conversion material having a high figure of merit, the heat conduction by the substrate lessens the temperature difference. The film thickness of the layer is at most several micrometers, whereas the thickness of the substrate is at least several hundred micrometers; thus, the performance deterioration due to the heat conduction by the substrate is a serious problem.
JP 62-177985A discloses a thermoelectric conversion element in which substrates made of glass or ceramic are stacked with an adhesive therebetween, the substrates having a p-type thermoelectric conversion layer formed on one surface thereof and an n-type thermoelectric conversion layer formed on the other surface.
JP 10-74987A discloses a thermoelectric conversion element in which a p-type thermoelectric conversion layer is formed on one surface of a film and an n-type thermoelectric conversion layer is formed on the other surface. As the film, a film made of a synthetic resin such as polyimide is disclosed in addition to a metal film.
The use of a substrate having a low thermal conductivity, such as glass or resin, can suppress the deterioration in thermoelectric conversion performance due to the heat conduction by the substrate. However, when resin or glass is made into a substrate, it is impossible to epitaxially grow a thermoelectric conversion layer on the substrate. Therefore, although heat conduction by the substrate can be suppressed, a problem remains that a thermoelectric conversion layer having good thermoelectric conversion performance cannot be formed.
Other than the above, JP 2002-316898A can be mentioned as a reference that relates to the present invention. Example 2 of this reference discloses that, by exposing a sapphire substrate on which gallium nitride is formed to water vapor, the gallium nitride and the sapphire substrate are separated.